(a) Field of the Invention
The present invention relates to a microscope objective and, more particularly, to a microscope objective wherein at least one element constituting the lens system is arranged as a graded refractive index lens (hereinafter referred to as a GRIN lens) formed that the refractive index thereof varies according to the radial distance from the optical axis.
(b) Description of the Prior Art
For microscope objectives, it is required to arranged that aberrations are corrected favourably and, at the same time, to make the numerical aperture (NA) large so as to obtain high resolving power. Besides, it is preferable to make the working distance (WD) long for the purpose of preventing the lens system from colliding against a specimen and for other purposes.
Moreover, as it is necessary to use various microscope objectives by exchanging them, microscope objectives are subject to restrictions, for example, the overall length of the lens system should e within a certain limit, the distance from the object to the image should be maintained as a constant value, and so forth. Furthermore, in ordinary microscope optical systems, blurring of colours is caused and the image becomes indistinct unless chromatic aberration is corrected satisfactorily favourably.
It is very difficult to concurrently satisfy all of the above-mentioned requirements also due to the fact that microscope objectives are enlarging lens systems.
To obtain a microscope objective which satisfies the above-mentioned requirements by overcoming the above-mentioned difficulties, it has been unavoidable to make the number o lenses constituting the lens system very large and, moreover, it has been necessary to use optical materials with anomalous dispersion such as fluorite etc. and to use many cemented lenses.
Generally, an optical system is constructed by combining a large number of lenses. To increase the aberration correcting capacity, it is known to use an aspherical lens and GRIN lens in addition to spherical lenses. Besides, as for correction of chromatic aberration, it is known to control not only refractive indices in the central portion of a lens in respect to wavelengths, i.e., dispersion, but also to control the distribution of refractive indices for wavelengths so as to thereby correct chromatic aberration.
Microscope optical systems each employing a GRIN lens are disclosed in Japanese published examined patent applications Nos. 28057/72 and 39405/82.
Out of said Japanese published examined patent applications, the former does not disclose any description related to correction of aberrations. The latter shows description about correction of offaxial aberrations by using a GRIN lens. However, it does not disclose any description about correction of chromatic aberration.